1. Technical Field of the Invention
The present invention relates generally to alternators. More particularly, the invention relates to an automotive alternator which has an improved structure for effectively cooling a field coil thereof.
2. Description of the Related Art
A conventional method of increasing the power output of an automotive alternator is to increase the magnetomotive force of a field coil of the alternator. Further, a possible way of increasing the magnetomotive force of the field coil is to reduce the impedance of the field coil. However, with continuous running of the alternator, the temperature of the field coil will increase due to heat generated by the field coil. Consequently, the resistance of the field coil will increase, thus reducing the magnetomotive force of the field coil.
Accordingly, it is necessary to effectively cool the field coil. However, the field coil is generally disposed in a center portion of a field core of the alternator, and the outer periphery of the field core is generally surrounded by a stator of the alternator. That is to say, the field coil is disposed in a place where it is difficult for the field coil to be effectively cooled by the cooling air flow created by a cooling fan of the alternator. In particular, when the field core is configured with a pair of lundell-type pole cores and there are further disposed a plurality of permanent magnets between the claws of the pole cores for improving the efficiency of the alternator, the vicinity of the field coil is so congested that it is difficult for the field coil to be exposed to the cooling air flow.
U.S. Pat. No. 5,233,255 discloses an automotive alternator, where the radial height of the pole cores is made greater than the axial length of the same, so as to increase the contact area between the pole cores and the field coil and thereby enhance the heat transfer from the field coil to the pole cores.
However, with the increase in the radial height of the pole cores, the outer diameter of the entire alternator accordingly increases, thus making the alternator difficult to be installed in the engine compartment of an automobile.
U.S. Pat. No. 4,418,295 discloses an automotive alternator, where a mixed flow fan is employed to create not only a radial air flow for cooling the armature coil but also an axial air flow for cooling the field coil.
However, when the number of turns and thus the outer diameter of the field coil is increased for the purpose of increasing the power output of the alternator or a plurality of permanent magnets are disposed between the claws of the pole cores as described above, it is still difficult for the field coil to be reliably exposed to the cooling air flows created by the cooling fan.